Methods of manufacturing a printed circuit board shielded against interfering radiation

ABSTRACT

Methods for the production of a board ( 11 ) with printed circuit ( 12 ) shielded against interfering radiation and having electronic components ( 14 ) comprise the steps of positioning of the electronic components ( 14 ) on contact points ( 15 ) designed for them, and the application of a shield ( 20 ), comprising a preformed metallized plastic film ( 30 ) over the top of the electronic components ( 14 ) and in electrical contact with the earth ( 13 ) on the board ( 11 ) with printed circuit ( 12 ), and also fixing the electronic components ( 14 ) on the board ( 11 ) with printed circuit ( 12 ) by means of an electrically conducting fixing agent ( 16 ); and fixing of the shield ( 20 ) on the board ( 11 ) with printed circuit ( 12 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national stage filing under 35 U.S.C. §371 ofPCT/NL01/00727, filed Oct. 3, 2001, which claims priority to NL 1016549,filed Nov. 6, 2000; and NL 1016354, filed Oct. 6, 2000, both of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method for the production of a boardwith printed circuit (printed circuit board) shielded againstinterfering radiation and having at least one electronic component.

It is customary for an electronic device to be protected, either inorder to protect the electronic device itself against interferingradiation coming from an external source or in order to protect theenvironment against radiation generated by the electronic device itself.

Electronic devices which may be sensitive to interfering radiation, suchas electromagnetic radiation, are electronic regulating, control andswitching devices, and likewise communications equipment and dataprocessing equipment, comprising one or more boards having a circuit onthem and provided with electronic components, which circuit comprises anetwork of electrical conductors. Examples of such devices comprise, forexample, microdata processors, computers, integrated circuits,microswitches, mobile telephones, transmitting and receiving equipment,pagers, television and so forth.

The term “shielding” is used in the art when referring to suchprotection of electronic equipment. The requirements which theabovementioned devices have to meet as regards such protection arebecoming increasingly strict, and to an increasing extent are being laiddown at international level.

Suitable shielding materials are generally composed of materials withgood electrical and/or magnetic conducting properties, and areconsequently often composed of metal. Examples of such shielding aremetal casings or plastic coverings provided with a thin metal layer andgenerally applied to the inside of a housing. Various techniques areknown for the application of such a metal layer directly to the insideof a housing or to a thin plastic film. Examples are metal painting,electroless plating, electroplating, vapour deposition and sputtering.In addition to these often complex, and therefore expensive techniques,in-mould foiling is a known process, in which either a metallizedplastic film is deformed in a mould, for example by deep drawing, or apreviously deformed metallized plastic film is placed in a mould, and inboth cases a molten resin is subsequently poured into the mould, inorder to place a carrier against the deformed metallized plastic film.Said carrier is often the housing or a part of the housing. Thistechnique and the shielding materials used in it are described in, forexample, WO99/40770 of the applicant.

EP-A-0,806,892 also discloses a production method, in which a shield isused, which shield consists of a plastic substrate which is firstdeformed and then metallized. Such a metallization of an alreadydeformed plastic substrate is, however, difficult to carry out. Theshield is fixed by means of conducting adhesive tape to an earth trackof a board with printed circuit.

Although some of the abovementioned techniques provide good shieldingresults, there is a need among manufacturers of the abovementionedelectronic equipment for simple shielding techniques, in particular fortechniques that can be used by the manufacturer himself during theassembly of the electronic equipment.

SUMMARY OF THE INVENTION

The present invention provides such techniques.

According to a first aspect of the invention, to that end the method forthe production of a board with printed circuit shielded againstinterfering radiation and having at least one electronic componentcomprises the steps of

-   a) the application of flowable electrically conducting fixing agent    to contact points for the at least one electronic component on the    board with printed circuit and to the earth for the shield;-   b) the positioning of the at least one electronic component on the    contact points concerned of the board with printed circuit;-   c) the positioning of a shield, comprising a preformed metallized    plastic film, over the top of the electronic component on the earth    of the board with printed circuit; and-   d) the simultaneous fixing of the at least one electronic component    on the contact points and the shield on the earth by increasing the    temperature to above the flowing point of the flowable electrically    conducting fixing agent.

In the case of this method according to the invention, a flowableelectrically conducting fixing agent, such as solder or conductingpolymer, for example in solid form, is first applied to the contactpoints and to an earth track. For the shielding, the flowable fixingagent can advantageously be applied in the form of a so-called microballgrid array. The contact points are in communication with the printedcircuit (network of electrical conductors). The one or more electroniccomponents are then placed, and the shield is subsequently placed overthe top of the electronic components. The temperature is then increasedto above the flow temperature of the fixing agent. In the case ofsoldering paste such a temperature increase can be carried out, forexample, in an oven by heating to 260° C. or more, depending on the typeof soldering paste used. The preformed shield retains its shape here.When the fixing agent has flowed sufficiently, the entire unit isallowed to cool down, so that solidification of the fixing agent cantake place and both the shield and the electronic components are therebyfirmly fixed on the PCB. The result of this is that both the electroniccomponents and the shield are fixed on the PCB in one step, instead ofbeing fixed in successive individual steps.

In the case of this method the shield is composed of a plastic film,which is moulded to the desired shape after metallization. After fixingof the shield, the latter is in electrical contact with a separate earthtrack of the PCB. This, as it were, forms a Faraday cage. These steps ofthis process are relatively simple and can easily be carried out by themanufacturer himself and integrated in existing production lines usingalready existing equipment. In addition, the shape of the shield can bekept relatively simple, and therefore so can the process for deformationof the metallized plastic film. This also gives an advantage comparedwith a shield comprising a preformed and subsequently metallized plasticsubstrate, such as, for example, according to EP-A-0,806,892. A simplerform of the ultimate shield also gives the advantage that the chance ofinterruptions or thickness differences to occur in the metal layerduring the deformation is slight.

A relatively simple shield advantageously comprises a substantiallybox-shaped element (also known as a shielding cap), the side wall(s) ofwhich is/are provided with a projecting edge extending parallel to themain surface of the PCB, preferably an outward extending side edge witha view to the prior deformation. The box-shaped element may be a round,rectangular or square container, or may have any other suitableappearance. Such a box-shaped element is easy to position on and fix tothe PCB by means of existing devices, for example the so-calledpick-and-place machines, robot-type machines which are capable ofplacing the correct components in the correct positions, and which arealready in use for positioning electronic components in the correctposition on the PCB.

Steps a) and b) are advantageously carried out by means of the alreadymentioned pick-and-place machines, in the case of this first variantpreferably with the same pick-and-place machine. In this way, a verysimple assembly is possible, in fact consisting of 2 phases, involvingpositioning, on the one hand, and fixing, on the other hand.

In the case of the method discussed above, the shield is also exposed toa high temperature, so that the flowable, electrically conducting fixingagent begins to flow. The plastic of the metallized plastic film istherefore advantageously a temperature-resistant material(preferably >260° C.), such as polyimide, PEI, PEEK, PES, polyamide andthe like, or a mixture of these, if desired in combination with lesstemperature-resistant materials, such as PC. The deformation of themetallized plastic film in order to produce the shield in the desiredshape from it is usually carried out at fairly high temperatures in therange between 100 and 300° C., in particular between 100 and 200° C.Thermoplastic plastics are therefore particularly suitable. A preferredmaterial for the plastic film is polyimide, on account of its goodtemperature resistance and deformation properties. Polyimide also hasproperties that are beneficial for the metallization itself. Theapplication of the thin metal layer to the plastic substrate forproducing the metallized plastic film can be carried out by, forexample, magnetron sputtering, thermal vacuum metallization, electrolessplating and the like. Polyimide does not exhibit any evaporationphenomena of monomer material from the polymer during vacuum processes.In the case of sputtering the plastic is preferablynon-halogen-containing, since in a conventional sputter unithalogen-containing material generally behaves aggressively(corrosively). If the thin metal layer is deposited on the plasticsubstrate by means of plating methods, the plastic is advantageouslyresistant to chemicals, in particular to acids.

The metal of the metallized plastic film is advantageously selected fromSn, Ni, Cu, Pb, alloys and mixtures thereof. A preferred materialcomprises tin or a tin alloy, on account of the good deformationproperties, as already recognized in the earlier mentioned publicationWO-A-98/40770. The properties mentioned in that publication, such asthicknesses of the plastic film and the layer of tin deposited on it, ifdesired with the interposition of a thin intermediate layer whichpromotes adhesion, or with a corrosion-resistant top layer, are likewiseusable in the case of the present invention.

An example of a suitable shielding material is a thermally stablepolyimide film deformable under vacuum, with a layer of tin 5 (+/−0.5)μm thick, which has a resistance (as R per square) of 0.18 +/−0.3ohm/square.

According to a further aspect, the invention relates to a method for theproduction of a board with printed circuit shielded against interferingradiation and having at least one electronic component, which methodcomprises the steps of

-   a) the positioning and fixing of the at least one electronic    component on contact points concerned of the board with printed    circuit;-   b) the positioning of a shield, comprising a preformed metallized    plastic film, over the top of the at least one electronic component    on the earth of the board with printed circuit, the shield being a    substantially box-shaped element, the side wall of which is provided    with a fixing edge extending parallel to the main surface of the    board, which fixing edge is provided with through apertures; and-   c) the fixing of the shield on the earth of the board with printed    circuit, using a fixing agent.

In the case of this variant of the method of the invention, in a firststep the electronic components are placed on the appropriate contactpoints of the PCB and fixed, for example with solder or conductingpolymer. The shield is then placed over the top of the components thuspositioned and is in electrically conducting contact with an earth trackpresent on the PCB. In the case of this method the shield used is madeof a plastic substrate that is first metallized and is then deformed toa shape desired for the shield. Such a shield has the advantagesmentioned earlier. In the case of this variant of the method the shieldis produced in the shape of a substantially box-shaped element. Such abox-shaped element comprises a bottom, vertical side walls and an opentop side. On the edges bounding the top side, the vertical side wallsare provided with fixing edges generally extending perpendicularly tothe side walls and facing outwards. In the fixed position, said fixingedges run parallel to the main surface of the PCB. Through apertures areprovided in said fixing edges, which apertures can act as parts forfixing the shield on the earth of the PCB. In a simple embodiment,adhesive tape which may or may not be conducting is used. Such adhesivetape comprises a (plastic) carrier, which is provided with an adhesivelayer on at least one side. The carrier can also comprise a plasticfoam, such as polyurethane foam, in particular polyurethane foam basedon polyether chains, on account of the compressibility and adaptability.The (single-sided) adhesive tape, which may or may not be conducting, isplaced at least partially on the side of the fixing edge facing awayfrom the PCB, and partially on the board with printed circuit by way ofthe apertures. A flowable fixing agent, such as solder and conductingpolymer, can also be used in the apertures. Other examples of such afixing agent comprise a mechanical fixing agent such as clips and thelike, interacting with holes or other fixing facilities provided in theboard with printed circuit. The use of adhesive tape permits simpleassembly.

A thermoplastic plastic can also be provided in the through apertures,which thermoplastic plastic flows when there is a temperature increaseand hardens when there is cooling, in order to produce a connectionbetween shield and board. It is important in that case for directelectrical contact to exist between the metal of the shield and themetal on the board. Direct electrical contact gives better conductance,and thus better protection. It also gives great durability.

The pick-and-place machines mentioned earlier can be used for carryingout the positioning of the electronic components and the shield. Withregard to the type of starting materials for the shield, namely theplastic and metal covering, and also the production techniques used,reference is made to the discussion of these given above. Temperatureresistance of the shield plays a subordinate role here.

In yet a further variant of the method for the production of a boardwith printed circuit shielded with a shield against interferingradiation and having at least one electronic component, said methodcomprises the steps of

-   a) the positioning and fixing of the at least one electronic    component on contact points concerned of the board with printed    circuit;-   b) the positioning of a shield, comprising a preformed metallized    plastic film, over the at least one electronic component on the    earth of the board with printed circuit; and-   c) the fixing of the shield on the earth of the board with printed    circuit, using a conducting, double-sided adhesive material, the    adhesion strength of the conducting, double-sided adhesive material    to the shield being greater than its adhesion strength to the earth    of the board with printed circuit.

In the case of this variant the fixing of the shield is carried outusing a conducting double-sided adhesive material, after the electroniccomponents have first been positioned on and fixed to, for examplesoldered on, the PCB. Since in the case of this variant the shieldingmaterial itself is not exposed to the high soldering temperatures, therequirements to be laid down for the shielding material as regardstemperature resistance are lower. For example, PC can be used. Ofcourse, the plastic materials mentioned above in the case of the firstvariant can also be used. The metal of the shielding material can beselected from the metals or their alloys already mentioned. In addition,the use of double-sided adhesive tape which is conducting has theadvantage that disassembly, for example for repair or recycling work, isrelatively easy to carry out. The adhesion strength of the adhesivematerial in strip form to the shield is greater than its adhesionstrength to the PCB, for example by applying a different adhesive layeron either side to a suitable strip-shaped substrate. The adjustment ofthe adhesion strength by means of an adhesion gradient in the thicknessdirection of the adhesive layer is likewise a possibility and is aprocedure which is known in the art concerned. The strip-shaped adhesivematerial is further advantageously applied to the fixing edges of thebox-shaped element, prior to the positioning of the shield on the PCB,because this is simpler than applying the strip-shaped adhesive materialto the PCB, which is more difficult to gain access to it after theelectronic components have already been fixed on it.

An example of a suitable conducting, strip-shaped double-sided adhesivematerial is an acrylic polymer containing a conducting metal, theconducting metal advantageously being selected from silver, nickel,copper, alloys and mixtures thereof; for example, with a thickness ofapproximately 0.05 mm, an elongation of approximately 21%, and aresistance of less than 0.02 ohm/inch.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below with reference to the appended drawing,in which:

FIG. 1 is a cross-sectional view of a first embodiment of a PCB with ashield acting against interfering radiation;

FIG. 2 is a cross section of a detail of a soldered connection betweenPCB and shield in FIG. 1;

FIG. 3 is a cross-sectional view of a second embodiment of a PCB with ashield acting against interfering radiation;

FIG. 4 is a cross section of a detail of the connection between PCB andshield in FIG. 3;

FIG. 5 is a cross-sectional view of a third embodiment of a PCB with ashield acting against interfering radiation; and

FIG. 6 is a cross section of a detail of the connection between PCB andshield in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a cross section of a first embodiment of a PCB shieldedagainst interfering radiation, indicated in its entirety by referencenumeral 10. Said PCB 10 comprises a rectangular carrier 11, on which asystem of electrical conductors 12 is provided as the printed circuit,which conductors are applied or printed in a manner that is usual in theart. A separate earth track 13 is further provided. Electroniccomponents 14 are provided at predetermined positions, here also calledcontact points 15, which components are fixed by means of solder 16 andare thus in contact with the conductors 12. A shield 20 is provided onthe PCB 10, in the embodiment shown said shield comprising a box 22 witha bottom 24 and side walls 26. At the open top side of the box 22, theside walls 26 are provided with outward extending fixing edges 28. Thebox 22 is formed from a metallized plastic film 30 by deep drawing atraised temperature, which film in the case illustrated comprises a metallayer 32 facing the PCB and a plastic substrate 34 (see FIG. 2), forexample a tin-plated polyimide film. If desired, a plastic filmmetallized on both sides can be used. The metal layer 32 is connected ina conducting manner to the earth track 13 by means of solder 16.

The embodiment illustrated in FIGS. 1 and 2 is made as follows. A smallquantity of solid solder 16 is applied to the contact points 15 on theboard 11 with network of conductors 12 and a separate earth track 13,and to various contact positions on the earth track 13 for the shield20. The electrical components 14 are then positioned on the contactpoints 15—for example, projecting contacts of the components 14 incorresponding recesses of the contact points. The shield 20 issubsequently positioned over the top of the electrical components 14.Both positioning steps are carried out with the same pick-and-placemachine. The assembly thus prepared is placed in an oven and heated toabove the flowing point of the solder 16. Both the components 14 and theshield are fixed firmly to the PCB 10 by allowing the solder to flow andsubsequently cool.

FIG. 2 shows in greater detail the soldered connection thus formedbetween the earth track 13 and the shield 20 using solder 16.

FIG. 3 shows another embodiment of a PCB shielded against interferingradiation. In this figure components corresponding to components fromFIGS. 1–2 are indicated by the same reference numerals. This embodimentof PCB 10 comprises a rectangular carrier 11, on which a system ofelectrical conductors 12 is provided as the printed circuit, whichconductors are applied or printed in a manner usual in the art. Aseparate earth track 13 is further provided. Electronic components 14are provided at predetermined positions, the contact points 15, whichcomponents are fixed by means of solder 16, and are thus in contact withthe conductors 12. A shield 20 is provided on the PCB 10, which shieldin the embodiment shown comprises a box 22 with a bottom 24 and sidewalls 26. On the open top side of the box 22, the side walls 26 areprovided with outward extending fixing edges 28. The box 22 is formed bydeep drawing at raised temperature from a metallized plastic film 30,which in the instance shown comprises a metal layer 32 facing the PCBand a plastic substrate 34 (see FIG. 4), for example a tin-platedpolyimide film. If desired, a plastic film metallized on both sides canbe used. A number of through apertures 29 are made in the fixing edge,and a single-sided adhesive tape strip 31 is provided on the fixing edge28, which adhesive tape is forced through the apertures 29 until it isin contact with the board 11. This means that the fixing requires littlesurface area of the carrier 11. The metal layer 32 in this way is indirect contact with the earth track 13, as can be seen in greater detailfrom the cross section according to FIG. 4.

The embodiment illustrated in FIGS. 3 and 4 is made as follows. Theelectrical components 14 are positioned on the contact points 15 on theboard 11 with network of conductors 12 and a separate earth track 13—forexample, projecting contacts of the components 14 in correspondingrecesses of the contact points—and soldered with solder 16. The shield20 is then positioned over the top of the electrical components 14, andfixed with the single-sided adhesive tape 31 through the apertures 29.If the shield 20 and adhesive tape 31 are resistant to high temperature,if desired the soldering step of the components 14 can be carried outafter positioning of both the components 14 and the shield 14, forexample by heating in an oven to above the flowing point of the solder16 and allowing to cool down. In that case it is easy to carry out bothpositioning steps with the same pick-and-place machine.

FIG. 4 shows in greater detail the electrical connection between theearth track 13 and the shield 20 when the single-sided adhesive tape 31is used.

FIG. 5 shows yet another embodiment of a PCB 10 shielded againstinterfering radiation. In this figure parts corresponding to parts fromFIGS. 1–2 are indicated by the same reference numerals. This embodimentof PCB 10 comprises a rectangular carrier 11, on which a system ofelectrical conductors 12 is provided as the printed circuit, whichconductors are placed or printed in a manner usual in the art. Aseparate earth track 13 is further provided. Electronic components 14are provided at predetermined positions, the contact points 15, whichare fixed by means of solder 16 and are thus in contact with theconductors 12. A shield 20 is provided on the PCB 10, which shield inthe embodiment illustrated comprises a box 22 with bottom 24 and sidewalls 26. On the open top side of box 22 the side walls 26 are providedwith outward extending fixing edges 28. The box 22 is formed from ametallized plastic film 30 by deep drawing at raised temperature, whichfilm in the case illustrated comprises a metal layer 32 facing the PCBand a plastic substrate 34 (see also FIG. 7), for example a tin-platedpolyimide film. If desired, a plastic film metallized on both sides canbe used. Strips of a conducting, double-sided adhesive tape 40 (see FIG.6) are provided on the fixing edges 28 of the box 22. After cooling ofthe assembly of PCB 10 and components 14, the box 22 prepared in thisway is positioned by means of a pick-and-place machine at its intendedposition and adhered.

The adhesive tape 40 is composed of a carrier layer 42 with a conductingadhesive layer 44, 46 respectively on either side, each layer having itsown adhesion strength, as explained above. The adhesive tape can also bemade of a conducting adhesive matrix containing metal particles, whichin the thickness direction is provided with an adhesion gradient, sothat its two surfaces have a different adhesion on the earth track 13and the carrier 11, and the shield 20 respectively.

1. Method for the production of a board with printed circuit shieldedagainst interfering radiation and having at least one electroniccomponent, which method comprises the following steps: a) thepositioning and fixing of the at least one electronic component oncontact points concerned of the board with printed circuit; b) thepositioning of a shield, comprising a preformed metallized plastic film,over the top of the electronic component on the earth of the board withprinted circuit, the shield being a substantially box-shaped element,the side wall of which is provided with a fixing edge extending parallelto the main surface of the board, which fixing edge is provided withthrough apertures; and c) the fixing of the shield on the earth of theboard with printed circuit by applying a fixing agent over the fixingedge and on the board with printed circuit by way of the apertures. 2.Method according to claim 1, wherein the fixing agent is single-sidedadhesive tape.
 3. Method according to claim 1, wherein the steps a) andb) are carried out with a so-called pick-and-place device.
 4. Methodaccording to claim 1, wherein the plastic of the metallized plastic filmis a temperature-resistant materials.
 5. Method according to claim 4,wherein the temperature-resistant material is selected from a groupconsisting of polyimide, PC, PEEK, PES, polyamide or mixtures thereof.6. Method according to claim 1, wherein the metal of the metallizedplastic film is selected from Sn, Ni, Cu, Pb, alloys and mixturesthereof.
 7. Method according to claim 6, wherein the metal comprisestin, lead or an alloy thereof.